<!doctype html>
<head>
	<meta charset="UTF-8">
	<title>Document</title>
</head>
<body>
	<script>
		function test(){
			//隐式操作
			//var str;
			console.log(str);//?
			var str = "Hello World";
			console.log(str);
		}
	</script>
	<button onclick="test()">点击</button>
</body>
</html>